kbuild: Do not remove vmlinux when cleaning external module

kbuild: Do not remove vmlinux when cleaning external module

Since commit 1f2bfbd00e466ff3489b2ca5cc75b1cccd14c123 "kbuild:
link of vmlinux moved to a script" make clean with M=<dir>
argument (so cleaning external module) removes vmlinux,
System.map and couple of other files from the *main* kernel
build directory! This not what was happening before and almost
certainly not what one would expect.

This patch moves makes the clean target of the script called
only when !KBUILD_EXTMOD.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Cc: stable@vger.kernel.org [v3.5+]
Signed-off-by: Michal Marek <mmarek@suse.cz>